Use HistogramDiff by default in DiffFormatter

Its behavior is similar to PatienceDiff, and runs nearly as fast,
often beating the performance of MyersDiff.

Change-Id: I43c3faefa8109f1a68ef57522bec9cf27b5df252
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/diff/DiffFormatter.java

@@ -118,7 +118,7 @@ public class DiffFormatter {
 
 	private int abbreviationLength = 7;
 
-	private DiffAlgorithm diffAlgorithm = MyersDiff.INSTANCE;
+	private DiffAlgorithm diffAlgorithm = new HistogramDiff();
 
 	private RawTextComparator comparator = RawTextComparator.DEFAULT;
 
@@ -213,7 +213,7 @@ public class DiffFormatter {
 	 *
 	 * @param alg
 	 *            the algorithm to produce text file differences.
-	 * @see MyersDiff#INSTANCE
+	 * @see HistogramDiff
 	 */
 	public void setDiffAlgorithm(DiffAlgorithm alg) {
 		diffAlgorithm = alg;